Which of the following qualifies as an allowable deduction in determining MAGI?

The student loan interest deduction is an allowable deduction when determining Modified Adjusted Gross Income (MAGI). This deduction allows individuals to deduct up to a certain amount of interest paid on qualified student loans, thereby reducing the amount of income considered for tax purposes. This can be particularly beneficial for those who are repaying their student loans as it decreases their tax burden and affects their eligibility for certain tax credits and deductions.

The other options do not qualify as deductions included in the MAGI calculation. The standard deduction is not subtracted when calculating MAGI, while the child tax credit is a tax credit and not a deduction. Similarly, mortgage interest is also a deduction, but it does not directly affect MAGI in the same way as the student loan interest deduction does, because only specific tax adjustments affect this calculation. Therefore, the student loan interest deduction stands out as the correct choice in determining MAGI.